1. Field of the Invention
The present invention relates to a stabilized optical pulse generator for generating a repetitive, high-frequency optical pulse that is required in constructing an optical transmission system which operates at an extremely high speed. The present specification is based on a patent application filed in Japan (Japanese Patent Application No. Hei 10-196374), a portion of which is incorporated herein.
2. Relevant Art
Optical pulse generators which can be applied to the aforementioned type of optical transmission system have, for example, been disclosed in Japanese Patent Application, First Publication No. Hei 8-18139. FIG. 5 is a block diagram showing a structural example of the optical pulse generator according to this document. This optical pulse generator comprises a ring resonator R in which a rare-earth doped optical fiber 1, optical branching circuit 2, optical isolator 3, optical modulator 4, optical filter 5, and optical multiplexer 6 are sequentially connected with optical fibers. At this point, the description will concentrate on the problematic aspects of the related art, with details of each component shown in the aforementioned figure being described later.
The ring resonator R generates a repetitive, high-frequency optical pulse by circulating a laser light. The optical branching circuit 2 branches a portion of the optical pulse circulating through the ring resonator R, and optical branching circuit 8 further branches the optical pulse branched by the aforementioned optical branching circuit 2, and sends a portion of this optical pulse to a clock signal extractor 9. This clock signal extractor 9 extracts a clock signal from the incident optical pulse. A phase shifter 13 then adjusts the phase of the extracted clock signal, after which an electrical amplifier 14 amplifies the output of the phase shifter 13 and outputs the resultant clock signal CLK to the optical modulator 4. Based on the clock signal CLK, the optical modulator 4 then generates an optical pulse after modulating the intensity of the laser light circulating through ring resonator R.
At this point, the clock signal extractor 9 detects the optical pulse that is branched from optical branching circuit 8 by means of photo-detector 10. The photo current flowing into the photo-detector 10 increases in conjunction with an increasing optical power of the incident optical pulse to photo-detector 10. As a result, the optical power of the optical pulse, which is branched from the optical branching circuit 8 to clock signal extractor 9, damages the aforementioned photo-detector 10 when the optical power exceeds the photo-detection capability of this photo-detector 10. In other words, it is necessary to employ the photo-detector 10 under conditions wherein a rated value of the photo-detection current is not exceeded, since damage to the aforementioned photo-detector 10 results when a photo-detection current exceeding this rated value flows into the photo-detector 10. However, the above-described conventional technology does not limit the optical power of the optical pulse received by the photo-detector 10 to below a rated value. Accordingly, when the photo-detector 10 is damaged, the extraction of the clock signal by the clock signal extractor 9 is no longer possible. As a result, the optical modulator 4 is not able to modulate the intensity of the laser light circulating through the ring resonator R, such that the generation of an optical pulse no longer occurs.